Re: [PATCH 1/3] Properly account for the number of page cache pages zone_reclaim() can reclaim

Date

Thu, 11 Jun 2009 20:37:04 +0900 (JST)

> On NUMA machines, the administrator can configure zone_reclaim_mode that> is a more targetted form of direct reclaim. On machines with large NUMA> distances for example, a zone_reclaim_mode defaults to 1 meaning that clean> unmapped pages will be reclaimed if the zone watermarks are not being met.> > There is a heuristic that determines if the scan is worthwhile but the problem> is that the heuristic is not being properly applied and is basically assuming> zone_reclaim_mode is 1 if it is enabled. The lack of proper detection can> manfiest as high CPU usage as the LRU list is scanned uselessly.> > Historically, once enabled it was depending on NR_FILE_PAGES which may> include swapcache pages that the reclaim_mode cannot deal with. Patch> vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch by> Kosaki Motohiro noted that zone_page_state(zone, NR_FILE_PAGES) included> pages that were not file-backed such as swapcache and made a calculation> based on the inactive, active and mapped files. This is far superior> when zone_reclaim==1 but if RECLAIM_SWAP is set, then NR_FILE_PAGES is a> reasonable starting figure.> > This patch alters how zone_reclaim() works out how many pages it might be> able to reclaim given the current reclaim_mode. If RECLAIM_SWAP is set> in the reclaim_mode it will either consider NR_FILE_PAGES as potential> candidates or else use NR_{IN}ACTIVE}_PAGES-NR_FILE_MAPPED to discount> swapcache and other non-file-backed pages. If RECLAIM_WRITE is not set,> then NR_FILE_DIRTY number of pages are not candidates. If RECLAIM_SWAP is> not set, then NR_FILE_MAPPED are not.> > [mmotm note: This patch should be merged with or replace> vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim. Kosaki?]> > [kosaki.motohiro@jp.fujitsu.com: Estimate unmapped pages minus tmpfs pages]> [fengguang.wu@intel.com: Fix underflow problem in Kosaki's estimate]> Signed-off-by: Mel Gorman <mel@csn.ul.ie>> Reviewed-by: Rik van Riel <riel@redhat.com>> Acked-by: Christoph Lameter <cl@linux-foundation.org>> ---> mm/vmscan.c | 55 +++++++++++++++++++++++++++++++++++++++++--------------> 1 files changed, 41 insertions(+), 14 deletions(-)> > diff --git a/mm/vmscan.c b/mm/vmscan.c> index 2ddcfc8..d832ba8 100644> --- a/mm/vmscan.c> +++ b/mm/vmscan.c> @@ -2333,6 +2333,44 @@ int sysctl_min_unmapped_ratio = 1;> */> int sysctl_min_slab_ratio = 5;> > +static inline unsigned long zone_unmapped_file_pages(struct zone *zone)> +{> + unsigned long file_mapped = zone_page_state(zone, NR_FILE_MAPPED);> + unsigned long file_lru = zone_page_state(zone, NR_INACTIVE_FILE) +> + zone_page_state(zone, NR_ACTIVE_FILE);> +> + /*> + * It's possible for there to be more file mapped pages than> + * accounted for by the pages on the file LRU lists because> + * tmpfs pages accounted for as ANON can also be FILE_MAPPED> + */> + return (file_lru > file_mapped) ? (file_lru - file_mapped) : 0;> +}> +> +/* Work out how many page cache pages we can reclaim in this reclaim_mode */> +static long zone_pagecache_reclaimable(struct zone *zone)> +{> + long nr_pagecache_reclaimable;> + long delta = 0;> +> + /*> + * If RECLAIM_SWAP is set, then all file pages are considered> + * potentially reclaimable. Otherwise, we have to worry about> + * pages like swapcache and zone_unmapped_file_pages() provides> + * a better estimate> + */> + if (zone_reclaim_mode & RECLAIM_SWAP)> + nr_pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);> + else> + nr_pagecache_reclaimable = zone_unmapped_file_pages(zone);> +> + /* If we can't clean pages, remove dirty pages from consideration */> + if (!(zone_reclaim_mode & RECLAIM_WRITE))> + delta += zone_page_state(zone, NR_FILE_DIRTY);

A percentage of the total pages in each zone. Zone reclaim will onlyoccur if more than this percentage of pages are file backed and unmapped.This is to insure that a minimal amount of local pages is still available forfile I/O even if the node is overallocated.

The default is 1 percent.

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but your code condider more addional thing. Can you please change document too?